In this mode, the rover's clock keeps running, but communications and other activities are suspended in order to put all available energy into heating and battery recharging. When the battery charge is adequate, the rover attempts to wake up and communicate on a schedule it knows.

"We may not hear from Spirit again for weeks or months, but we will be listening at every opportunity, and our expectation is that Spirit will resume communications when the batteries are sufficiently charged," said John Callas of NASA's Jet Propulsion Laboratory, Pasadena, Calif., who is project manager for Spirit and its twin rover, Opportunity....

Spirit had been communicating on a once-per-week schedule in recent weeks. During the designated time for the rover to communicate with NASA's Mars Odyssey orbiter passing overhead on March 30, Odyssey heard nothing from the rover.

"We are checking other less-likely possibilities for the missed communication, but this probably means that Spirit tripped a low-power fault sometime between the last downlink on March 22 and yesterday," Callas said. "The recent downlinks had indicated that the battery state of charge was decreasing, getting close to the level that would put Spirit into this hibernation."Winter Solstice is not until May 13, more than six weeks from now. So it doesn't seem likely to me (though this is just my own speculation) that we'll hear from her again for three months at least, into July, or even August.